1. Field of the Invention
This invention relates to zoom lenses and, more particularly, to zoom lenses having ghost and flare reduced for good optical performance suited to photographic cameras or video cameras.
2. Description of the Related Art
For the photographic cameras, video cameras, etc., zoom lenses of lage relative aperture and high range with high optical performance are demanded.
Of these, in the video camera, because its image pickup element is relatively low in sensitivity, the aperture ratio of the zoom lens is required to be as large as possible.
Besides this, in the color cameras using the single-tube type color image pickup tube or the image pickup plate such as a CCD along with the stripe filter, it is required to design the optical system as a whole in a telecentric form such that a light beam incident on the stripe filter is as normal as possible relative to the plane of the stripe filter in order to prevent mixture of colors.
Also, as, today, the CCD or MOS has been widely used as the image pickup element of the video camera, the surface of its cover glass and the surface of the image pickup element have generally high reflectance. For this reason, light reflected from these surfaces is reflected from the lens surfaces of the photographic lens or from the lens barrel, etc., and then enters the image pickup element again, so that there is a cause of producing the so-called ghost and flare.
As a technique that has a good image quality obtained from the attention to this point, there is U.S. patent application Ser. No. 223,108 filed on July 22, 1988.
In this proposal, with respect to each lens surface lying on the image plane side of the diaphragm, on the assumption that the light rays which has returned back from the image plane are reflected therefrom, the ray tracing operation is carried out, so that even if the light rays reflected from the lens surfaces form an image, the image takes its place on the photographic lens side of the predetermined image plane when bending of each lens is performed. Thus, the radius of curvature of each lens is determined to thereby remove harmful light. It should be noted that the reason why this has been done on the more rear lens surfaces than the diaphragm is that when the diaphragm is open at full aperture, because of the presence of light rays of various angles, the influence of the harmful rays becomes uniform over the entirety, being not too much conspicuous. Therefore, no problem arises on actual practice. On the other hand, when the diaphragm is stopped down, the influence gets appreciable. Hence attention is paid to that part which causes reflection of light when the diaphragm is stopped down.
By the way, as the size of aperture opening of the diaphragm nears the minimum, an appreciable diffraction phenomenon takes place due to the influence of the diaphragm blades. Hence, the image quality starts to deteriorate. Therefore, to avoid this phenomenon, an ND filter fulfills the light reducing function is put into the arrangement when the stopping down is started, thus permitting an exposure to be made with the diaphragm in a relatively wide open state.
However, the use of this filter gives rise to a new problem. That is, if this filter is positioned, for example, in a section constituting an afocal optical system, the light beam reflected from the image pickup element is reflected again by this filter to re-focus at a point near the image pickup element, thus forming ghost.
Meanwhile, as the zoom lenses suited to video cameras, mention may be made of U.S. Pat. Nos. 4,618,219, 4,621,905, 4,659,187, 4,653,874 and 4,518,228. Besides these, there are Japanese Laid-Open Patent Application No. Sho 59-222807, Japanese Laid-Open Patent Application No. Sho 60-260912, Japanese Laid-Open Patent Application No. Sho 61-20291, and U.S. patent application Ser. No. 190,472 filed on May 5, 1988, now U.S. Pat. No. 4,812,024.